In most places an individual can't be compelled to do much of anything for a client or employer regardless of what a contract says, since generic laws against slavery and similar concepts will apply. This is true almost everywhere in the first world, whether or not it has at-will employment. Normally there are some special legal provisions for cases where you need this not to apply, such as the military or emergency services.

However, it is entirely possible that a client could successfully claim back much of the compensation they paid for the original work via a civil suit if they subsequently argued that the original fee included maintenance and that maintenance was not in fact being carried out.

If it could be argued that the lack of maintenance also resulted in other damage to the client, they could potentially go after compensation for that damage as well. In the absence of a written contract putting clear (and legally enforceable) limits on such things, that could be far more expensive than just paying back the original revenues.

As an aside, this is particularly dangerous for freelancers, because if you don't have a written agreement making clear who the client was actually contracting to do the work, a court might find that it was the freelancer personally who was party to the agreement and not any corporate entity they normally work through. That would leave no legal shield to protect the freelancer from losing everything they personally own to settle a severe damages award.